Linear motors are widely used for a variety of applications. In one group of applications, high speed, or accurate waveform tracking, and accurate positioning are required from the linear motor. Stable operation without overshoot or oscillation is also a requirement. An example of such an application is a linear motor used to drive the tuning plunger in a high frequency, high power magnetron oscillator. The magnetron oscillator is commonly used in a radar transmitter, and the tuning plunger controls the transmitted frequency. The transmitted frequency may be varied in accordance with different frequency control waveforms, including rapid steps between frequencies and gradual frequency variations. In such an application, the speed, position accuracy and stability of the linear motor which drives the tuning plunger are of utmost importance.
Linear motors for such applications typically include a circumferential coil on a cylindrical support which is rigidly attached to a linearly movable shaft, and means for producing a radial magnetic field which intersects the conductors of the coil. When a current is applied to the coil, the magnetic field exerts an axial force on the coil, causing linear motion.
It is customary to place the linear motor in a servo loop to provide well-controlled operation. The position of the movable shaft is sensed by a position sensor and fed back for comparison with a desired position signal. An error signal produced by the comparison is supplied to an amplifier for energizing the coil to correct the position of the shaft. It is well-known that the bandwidth of a servo loop, which determines the rate at which the shaft position can be changed, depends on the loop gain. However, when servo loop has high closed loop gain, the tendency for instability and oscillation is increased. A second servo loop, where velocity is measured and applied to the amplifier as a negative or damping feedback, can be used to control oscillation.
A second limitation to the usable gain occurs in the structure of the motor. The motor force element is connected mechanically to a position sensor and a velocity sensor through a structure which exhibits resonant characteristics. Thus, sensors which normally return a negative feedback signal to the servo can return an amplified in-phase signal to the servo loop, thereby causing oscillation. The secondary resonance cannot be permitted to function since it is damaging to the motor structure and consumes large amounts of servo power. It is not necessary that the servo and force element be separated by the secondary resonance element as long as the secondary resonance can introduce a large signal into the sensor.
The simplest solution to the secondary resonance problem is to reduce the forward gain until the secondary oscillation ceases. This also reduces the effect of servo bandwidth and directly reduces servo performance. Another solution is to use an electronic notch filter in the feedback loop to suppress oscillations at the secondary resonance frequency. This solution is superior to the reduction of gain, but also tends to reduce bandwidth since the filter is likely to exhibit a broad continuation of gain over a wide bandwidth, causing a reduction in the desired operating range. Mechanical damping can also be used but adds mass which reduces servo performance.
It is a general object of the present invention to provide a novel wide bandwidth linear motor system.
It is another object of the present invention to provide a novel linear motor system wherein secondary resonances are suppressed.
It is a further object of the present invention to provide a novel wide bandwidth linear motor system wherein secondary resonances are suppressed without adversely affecting motor performance.
It is a further object of the present invention to provide a linear motor having a coil support which is subdivided into separate coil support elements for suppression of secondary resonances.